Output device and electronic device provided with same

ABSTRACT

A switching power supply IC ( 21 ) is provided with, in a form integrated into an integrated circuit: a terminal (T 2 ) whereupon an input voltage (Vin) is applied through an inductor (Lex); a terminal (T 3 ) from which an output voltage (Vout) for a loading LED is extracted; a transistor (N 1 ) connected between the terminal (T 2 ) and a grounding end; a transistor (P 1 ) connected between the terminal (T 2 ) and the terminal (T 3 ); a transistor (P 2 ) connected between the terminal (T 3 ) and a backgate of the transistor (P 1 ); and a switching control section (CTRL) for performing switching control to each transistor. During a step-up operation, the switching control section (CTRL) controls the switching of the transistors (N 1 , P 1 ) complementarily to each other while continuously keeping the transistor (P 2 ) on, and turns off all of the transistors (N 1 , P 1 , P 2 ) when a step-up operation is stopped.

TECHNICAL FIELD

The present invention relates to a voltage step-up type switching powersupply device (a chopper type power supply device) that steps up aninput voltage to generate an output voltage.

BACKGROUND ART

As shown in FIG. 6A, in a conventional voltage step-up type switchingpower supply IC (Integrated Circuit) 100, the current let flow throughan externally fitted inductor Lex is controlled by the switching of apower transistor PT integrated in the IC, and an output voltage Vout tobe fed to a load Z is generated by rectifying-smoothing means composedof an externally fitted Schottky-barrier diode SBD and an externallyfitted smoothing capacitor Cex.

As conventional arts related to the present invention, variousdisclosures and proposals have been made, as exemplified by: asemiconductor device capable of preventing parasitic-diode-associatedfaults that may invite various problems in the circuit built with thesemiconductor device (see Patent Publication 1); a rectifier circuitcapable of, even in cases where a MOSFET is used as a rectifier device,reducing the voltage fall across the MOSFET and thereby preventingdegradation of rectification efficiency (see Patent Publication 2); anda power supply circuit capable of reducing the area occupied by theoutput-stage circuit thereof and thereby easily preventing a reversecurrent from flowing from a capacitor that is connected to an outputterminal in parallel with a load (see Patent Publication 3).

-   Patent Publication 1: JP-A-H08-186261 (FIG. 3)-   Patent Publication 2: JP-A-H11-233730-   Patent Publication 3: JP-A-2003-347913

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

It is true that the above described conventional voltage step-up typeswitching power supply IC 100 is capable of stepping up an input voltageVin to generate an output voltage Vout by on/off control (duty control)of the power transistor PT.

Inconveniently, however, in the conventional voltage step-up typeswitching power supply IC 100 shown in FIG. 6A, on the occasion that theoperation of the device is stopped, even when the switching of the powertransistor PT is stopped, the current path from the node to which theinput voltage Vin is applied to the load Z is not necessarily cut off,and thus, for example, a variation in the input voltage Vin may cause aleak current to flow through the load Z. Disadvantageously, this resultsin an incomplete turning-off of power.

A solution to the above described disadvantage is to additionallyprovide, as shown in FIG. 6B, an externally fitted transistor Qex forcutting off the above mentioned current path. Doing so, however,involves the additional mounting of the transistor Qex, and thus makesit difficult to make the electronic device that incorporates the voltagestep-up type switching power supply IC 100 compact, light, and slim as awhole.

Even with the externally fitted transistor Qex additionally provided asdescribed above, if the node from which the output voltage Vout is fedto the load Z short-circuits to a low-potential node (ground short) asshown in FIG. 6C, a large current flows through the inductor Lex and theSchottky-barrier diode SBD and destroys these devices.

None of the conventional arts disclosed and proposed in the abovementioned Patent Publications 1 to 3 makes it possible to properly cutoff the current path from the node to which the input voltage Vin isapplied to the load Z, and thus none of them helps to solve thedisadvantage described above.

An object of the present invention is to provide a voltage step-up typeswitching power supply device capable of properly cutting off thecurrent path from a node to which an input voltage is applied to a loadas necessary, and thereby to contribute to achieving miniaturization ofand improved reliability in an electronic device incorporating such avoltage step-up type switching power supply device.

Means for Solving the Problem

To achieve the above object, according to one aspect of the presentinvention, a voltage step-up type switching power supply device isprovided with, in a form integrated into an integrated circuit, an inputterminal to which an input voltage is applied via an externally fittedinductor, an output terminal from which an output voltage is fed to aload, an output transistor connected between the input terminal and apredetermined standard voltage node, a first P-channel field-effecttransistor connected between the input terminal and the output terminal,a second P-channel field-effect transistor connected between the outputterminal and a backgate of the first P-channel field-effect transistor,and a switching control section that controls switching of the outputtransistor, the first P-channel field-effect transistor, and the secondP-channel field-effect transistor. Here, the switching control sectionis configured such that, when the input voltage is stepped up togenerate the output voltage, the switching control section controlsswitching of the output transistor and the first P-channel field-effecttransistor complementarily to each other while keeping the secondP-channel field-effect transistor on, and, when the operation of thedevice is stopped, the switching control section turns off all of theoutput transistor, the first P-channel field-effect transistor, and thesecond P-channel field-effect transistor (first configuration).

With this configuration, not only a current path via the first P-channelfield-effect transistor but also a current path via a body diodeaccompanying the first P-channel field-effect transistor can be cut offby the second P-channel field-effect transistor. Therefore, unlike theconventional configuration where an externally fitted transistor isprovided for cutting off a leak current, this configuration makes itpossible to properly cut off a current path from the node to which theinput voltage is applied to the load as necessary without preventingminiaturization and weight/thickness reduction of an electronic deviceincorporating the voltage step-up type switching power supply device.

According to the present invention, it is preferable that the voltagestep-up type switching power supply device having the firstconfiguration described above be further provided with, in a formintegrated into the integrated circuit, a power supply terminal to whichthe input voltage is directly applied and a third P-channel field-effecttransistor whose source is connected to the backgate of the firstP-channel field-effect transistor and whose drain is connected to thepower supply terminal, a gate of the third P-channel field-effecttransistor being connected to the output terminal (secondconfiguration). With this configuration, it is possible, when the outputvoltage falls to a predetermined potential level, to firmly fix backgatevoltages of the first and second P-channel field-effect transistors tothe input voltage and thereby to ensure that the first and secondP-channel field-effect transistors remain in the off state.

According to the present invention, it is preferable that the voltagestep-up type switching power supply device having the firstconfiguration described above be configured such that, when theoperation of the device is stopped, the switching control sectionapplies the input voltage to gates of the first and second P-channelfield-effect transistors (third configuration). With this configuration,the output voltage is allowed to fall abruptly. With this configuration,it is also possible to prevent a voltage which appears at the inputterminal from abruptly rising at the same time that the operation of thedevice is stopped and thereby to eliminate the need to provide thedevice with an unnecessarily large withstand voltage margin.

According to the present invention, it is preferable that the voltagestep-up type switching power supply device having the secondconfiguration described above be configured such that, when theoperation of the device is stopped, the switching control sectionapplies the input voltage to gates of the first and second P-channelfield-effect transistors (fourth configuration). With thisconfiguration, while the first and second P-channel field-effecttransistors are on, the third P-channel field-effect transistor can bekept off and backgates of the first and second P-channel field-effecttransistors can be kept in a floating state, and thus it is possible toprevent a large current from flowing via the third P-channelfield-effect transistor into internal circuits to which power issupplied via the power supply terminal and thereby to protect theircomponents from breakdown.

According to the present invention, it is preferable that the voltagestep-up type switching power supply device having either the first orsecond configuration described above be configured such that, when theoperation of the device is stopped, the switching control sectionapplies the output voltage to gates of the first and second P-channelfield-effect transistors (fifth configuration). Thus, in cases where itis necessary to turn on/off the switching power supply IC 21 many times,adopting this configuration makes it possible to reduce power loss inand improve efficiency of the device.

According to the present invention, it is preferable that the voltagestep-up type switching power supply device having any one of the firstto fifth configurations described above be further provided with, in aform integrated into the integrated circuit, an error amplifier thatgenerates an error voltage signal by amplifying a difference between apredetermined target voltage and a feedback voltage that varies with theoutput voltage, an oscillator that generates a predeterminedtriangular-wave voltage signal, and a PWM comparator that compares theerror voltage signal with the triangular-wave voltage signal andgenerates a PWM signal. Here, when the input voltage is stepped up togenerate the output voltage, the switching control section, based on thePWM signal, controls the switching of the output transistor and thefirst transistor complementarily to each other (sixth configuration).With this configuration, it is possible to keep the output voltage equalto its target value.

According to the present invention, it is preferable that the voltagestep-up type switching power supply device having any one of the firstto sixth configurations described above be further provided with anoutput-short detecting circuit that monitors the output voltage andgenerates an output-short detection signal for indicating to theswitching control section whether or not an output shout circuit isoccurring. Here, the switching control section, when it recognizes thatan output short is occurring based on the output-short detection signal,turns off all of the output transistor, the first P-channel field-effecttransistor, and the second P-channel field-effect transistor (seventhconfiguration). With this configuration, it is possible to prevent ashort-circuit current from flowing from the inductor into the switchingpower supply device and thereby to protect devices inside the device andcomponents externally fitted to the device from breakdown.

According to the present invention, it is preferable that, in thevoltage step-up type switching power supply device having the seventhconfiguration described above, the output-short detecting circuit beprovided with a comparator that inverts an output logic thereof when theoutput voltage falls below a predetermined threshold voltage, andoutput-detection masking means that, during a predeterminedoutput-detection masking period starting at a startup of the device,produces a constant output regardless of the output logic of thecomparator, and after the predetermined output detection masking periodends, reflects the output logic of the comparator by inverting theoutput logic of the output-short detection signal. Here, the switchingcontrol section, on recognizing that an output short is occurring basedon the output-short detection signal, turns off all of the outputtransistor, the first P-channel field-effect transistor, and the secondP-channel field-effect transistor (eighth configuration). With thisconfiguration, it is possible to prevent failure to start up which maybe caused by the comparator being incapable of output detection.

According to another aspect of the present invention, an electronicdevice is provided with a voltage step-up type switching power supplydevice having any one of the first to eighth configurations describedabove as output conversion means for converting an output of a devicepower source. Here, as the device power source, battery may be used.With this configuration, it is possible to achieve miniaturization andimproved reliability of the electronic device.

Advantages of the Invention

As discussed above, with a voltage step-up type switching power supplydevice of the present invention, it is possible to properly cut off thecurrent path from the node to which the input voltage applied to theload, if necessary, and thereby to contribute to miniaturization andimproved reliability of the electronic device incorporating the same.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] A block diagram showing a mobile-phone unit embodying thepresent invention;

[FIG. 2] A circuit diagram showing an example of the configuration of aDC/DC converter 20;

[FIG. 3] A diagram illustrating an example of the shut-down operation ofa switching power IC 21;

[FIG. 4] A diagram illustrating another example of the shut-downoperation of the switching power IC 21;

[FIG. 5] A diagram illustrating an example of the output-shortprotection operation of the switching power IC 21;

[FIG. 6A],

[FIG. 6B], and

[FIG. 6C] Circuit diagrams showing an example of a conventional DC/DCconverter.

LIST OF REFERENCE SYMBOLS

-   -   10 battery    -   20 DC/DC converter (switching regulator)    -   30 TFT liquid crystal panel    -   21 switching power supply IC    -   211 switch drive circuit    -   212 output feedback circuit    -   213 output-short (ground short) detecting circuit    -   CTRL switching control section    -   N1 to N3 N-channel field-effect transistors    -   P1 to P3 P-channel field-effect transistors    -   ERR error amplifier    -   E1 DC voltage supply    -   OSC oscillator    -   CMP1, CMP 2 comparators    -   I1 constant current supply    -   C1 capacitor    -   R1 to R3 resistors    -   SW switch device    -   INV inverter    -   OR OR circuit    -   T1 to T4 external terminals    -   Lex inductor (externally fitted)    -   Cex smoothing capacitor (externally fitted)    -   Rex resistor (externally fitted)

BEST MODE FOR CARRYING OUT THE INVENTION

The following description discusses examples of how the presentinvention is applied to a DC/DC converter incorporated in a mobile-phoneunit for the purpose of converting the output voltage of a battery togenerate a drive voltage for driving different parts (for example, a TFT(thin film transistor) liquid crystal panel) of the mobile-phone unit.

FIG. 1 is a block diagram showing a mobile-phone unit embodying thepresent invention (in particular, the power supply system for the TFTliquid crystal panel). As shown in the figure, the mobile-phone unit ofthis embodiment is provided with a battery 10 which is a device powersource, a DC/DC converter 20 which is output converting means of thebattery 10, and a TFT liquid crystal panel 30 which is display means ofthe mobile-phone unit. Although not shown in the figure, it should beunderstood that the mobile-phone unit of this embodiment is providedwith, in addition to the above mentioned components, atransmitter-receiver circuit portion, a speaker portion, a microphoneportion, a display portion, an operation portion, a memory portion, andthe like as means for realizing its essential functions (communicationfunction, etc.).

The DC/DC converter 20 generates a constant output voltage Vout from aninput voltage Vin applied by the battery 10, and then supplies theoutput voltage Vout to the TFT liquid crystal panel 30.

FIG. 2 is a circuit diagram (partly presented as a block diagram)showing an example of the configuration of the DC/DC converter 20. Asshown in this figure, the DC/DC converter 20 of this embodiment is avoltage step-up type switching regulator (chopper type regulator)having, in addition to a switching power supply IC21, an externallyfitted inductor Lex, a smoothing capacitor Cex, and a resistor Rex.

The switching power supply IC 21 is provided with, in terms of circuitblocks, a switch drive circuit 211, an output feedback circuit 212, andan output-short (ground short) detecting circuit 213, and is alsoprovided with external terminals T1 to T4 as external electricalconnection means. In the switching power supply IC 21, in addition tothe above mentioned circuit blocks, a protection circuit block (alow-input malfunction prevention circuit, a heat protection circuit, orthe like) may additionally be incorporated, if necessary.

The switch drive circuit 211 includes a switching control section CTRL,an N-channel field-effect transistor N1, and P-channel field-effecttransistors P1 to P3.

The output feedback circuit 212 includes an error amplifier ERR, a DCvoltage supply E1, an oscillator OSC, and a comparator CMP1.

An output-short (ground short) detecting circuit 213 includes acomparator CMP2, a constant current supply I1, a capacitor C1, a switchdevice SW, an inverter INV, N-channel field-effect transistors N2 andN3, resistors R1 to R3, and an OR circuit OR.

The power input node of the switching control section CTRL and the powerinput nodes of other internal circuits (not shown) are all connected tothe external terminal T1 (power supply terminal).

The drain of the transistor N1 is connected to the external terminal T2(input terminal). The source of the transistor N1 is grounded. The gateof the transistor N1 is connected to the gate-signal output node of theswitching control section CTRL.

The drain of the transistor P1 is connected to the external terminal T2.The source of the transistor P1 is connected to the external terminal T3(output terminal). The gate of the transistor P1 is connected to thegate-signal output node of the switching control section CTRL. Thetransistor P1 is accompanied by, between the drain and backgate thereof,a body diode BD1, with the anode and cathode thereof connected to thedrain and backgate of the transistor P1, respectively.

The drain of the transistor P2 is connected to the external terminal T3.The source of the transistor P2 is connected to the backgate of thetransistor P1. The gate of the transistor P2 is connected to thegate-signal output node of the switching control section CTRL. Thebackgate of the transistor P2 is connected to the source of thetransistor P2 itself. The transistor P2 is accompanied by, between thedrain and backgate thereof, a body diode BD2, with the anode and cathodethereof connected to the drain and backgate of the transistor P2,respectively.

The drain of the transistor P3 is connected to the external terminal T1.The source of the transistor P3 is connected to the backgate of thetransistor P1. The gate of the transistor P3 is connected to theexternal terminal T3. The backgate of the transistor P3 is connected tothe source of the transistor P3 itself.

The inverting input terminal (−) of the error amplifier ERR is connectedto the external terminal T4 (output feedback terminal). Thenon-inverting input terminal (+) of the error amplifier ERR is connectedto the positive end of the DC voltage supply E1. The negative end of theDC voltage supply E1 is grounded. The non-inverting input terminal (+)of the comparator CMP1 is connected to the output terminal of the erroramplifier ERR. The inverting input terminal (−) of the comparator CMP1is connected to the output end of the oscillator OSC. The outputterminal of the comparator CMP1 is connected to the PWM signal inputnode of the switching control section CTRL.

The non-inverting input terminal (+) of the comparator CMP2 is connectedto the external terminal T3. The inverting input terminal (−) of thecomparator CMP2 is connected to the connection node between theresistors R2 and R3, which are connected in series between the powerline and the ground end. The output terminal of the comparator CMP2 isconnected to one input node of the OR circuit OR. One end of the switchdevice SW is connected to the power line via the constant current supplyI1. The other end of the switch device SW is grounded via the capacitorC1 and is also connected to the gate of the transistor N2 and to thedrain of the transistor N3. The control terminal of the switch device SWis connected to the startup signal Sstart application node. The drain ofthe transistor N2 is connected to the power line via the resistor R1 andis also connected to the other input node of the OR circuit OR. Thesources of the transistors N2 and N3 are grounded. The gate of thetransistor N3 is connected, via the inverter INV, to the startup signalSstart application node. The output node of the OR circuit OR isconnected to the detection signal input node of the switching controlsection CTRL.

Outside the switching power supply IC 21, the external terminal T1 isconnected to the node to which input voltage Vin is applied from thebattery 10, and is also connected to one end of the inductor Lex. Theexternal terminal T2 is connected to the other end of the inductor Lex.The external terminal T3 is grounded via the smoothing capacitor Cex,and is also connected to the anode end of a light-emitting-diode lineLED of the TFT liquid crystal panel 30 serving as a load. The externalterminal T4 is grounded via the resistor Rex, and is also connected tothe cathode end of the light-emitting-diode line LED.

Now, a description will be given of the basic operation (DC/DCconversion operation) of the switching power supply IC 21 configured asdescribed above.

The transistor Ni is an output transistor of which the switching(turning-on and -off) is controlled according to the gate voltage Vnfrom the switching control section CTRL, and the transistor P1 is asynchronous rectifier transistor of which the switching (turning-on and-off) is controlled according to the gate voltage Vp1 from the switchingcontrol section CTRL.

The switching control section CTRL, when the input voltage Vin isstepped up to generate the output voltage Vout, controls the switchingof the transistors N1 and P1 complementarily to each other with thetransistor P2 constantly kept on.

It should be understood that the term “complementarily” used in thisspecification covers not only cases where the turning on/off of thetransistor N1 and that of the transistor P1 take place exactlyoppositely to each other but also cases where, from the perspective ofpreventing a through current, the turning on/off of the transistor N1takes place with a predetermined delay relative to that of thetransistor P1.

When the transistor N1 is turned on, a switch current flowing via thetransistor N1 to the ground end flows through the inductor Lex, and theelectric energy of the switch current is stored in the inductor Lex. Inthe on-period of the transistor N1, if the smoothing capacitor Cex hasalready been charged, a current from the smoothing capacitor Cex flowsthrough the light emitting diode line LED serving as a load. In thisstate, the transistor P1, which is a synchronous rectifier device, isturned off complementarily to the on-state of the transistor N1, andhence no current is allowed to flow into the transistor N1 from thesmoothing capacitor C1.

On the other hand, when the transistor N1 is turned off, a counterelectromotive voltage appears in the inductor Lex to discharge theelectric energy that has been stored therein. In this state, thetransistor P1 is turned on complementarily to the off-state of thetransistor N1, and thus the current passed from the external terminal T2via the transistor P1 is allowed to flow from the external terminal T3into the light emitting diode line LED serving as the load, and is alsoallowed to flow into the ground end via the smoothing capacitor Cex,thereby charging the smoothing capacitor Cex. Through the repetition ofthe above described operation, a DC output smoothed by the smoothingcapacitor Cex is supplied to the light emitting diode line LED servingas the load.

Thus, the switching power supply IC 21 of this embodiment, bycontrolling the switching of the transistors N1 and P1, serves as acomponent of a chopper type voltage step-up circuit that steps up theinput voltage Vin to generate the output voltage Vout.

Next, a description will be given of the output feedback controloperation of the switching power supply IC 21 configured as describedabove.

In the switching power supply IC 21, the error amplifier ERR amplifiesthe difference between an output feedback voltage Vfb (corresponding tothe actual value of the output voltage Vout) derived from one end of theresistor Rex and a reference voltage Vref (corresponding to the targetvalue Vtarget of the output voltage Vout) generated by the DC voltagesupply E1, thereby generating an error voltage signal Verr. Therefore,the further the output voltage Vout is lower than its target valueVtarget, the higher the voltage level of the error voltage signal Verris. The oscillator OSC generates a predetermined triangular-wave voltagesignal Vslope.

The comparator CMP1 is a PWM comparator that compares the error voltagesignal Verr and the triangular-wave voltage signal Vslope and generatesa PWM (pulse width modulation) signal. More specifically, the on-duty ofthe PWM signal (the rate of the on-period of the transistor N1 per unitperiod) continuously varies according to which of the error voltagesignal Verr and the rectangular-wave voltage signal Vslope is higher.More specifically, the further the output voltage Vout is lower than thetarget voltage thereof Vtarget, the larger the on-duty of the PWM signalis; and the closer the output voltage Vout is to the target voltagethereof Vtarget, the smaller the on-duty of the PWM signal is.

The switching control section CTRL, when the input voltage Vin isstepped up to generate the output voltage Vout, controls the switchingof the transistors N1 and P1 complementarily to each other in responseto the PWM signal. More specifically, the switching control section CTRLkeeps the transistor N1 on and the transistor P1 off during theon-period of the PWM signal; the switching control section CTRL keepsthe transistor N1 off and the transistor P1 on during the off-period ofthe PWM signal.

Thus, the switching power supply IC 21 of this embodiment is capable ofkeeping the output voltage Vout equal to its target value Vtarget by anoutput feedback control operation based on the error voltage signalVerr.

Next, a detailed description will be given of how the operation of theswitching power supply IC 21 configured as described above is stopped(the shut-down operation thereof) with reference to FIGS. 2 and 3. FIG.3 is a diagram illustrating an example of the shut-down operation of theswitching power supply IC 21, and particularly shows the device behaviorobserved when an input voltage Vin is applied to the gates of thetransistors P1 and P2 to stop the operation of the device.

When a shut-down instruction is given to the switching power supply IC21 at time t1, a ground voltage is applied to the transistor N1 as thegate voltage Vn, and the transistor N1 is turned off. Thus, theswitching power supply IC 21 stops its voltage step-up operation.

On the other hand, the input voltage Vin is applied to each of thetransistors P1 and P2 as gate voltages Vp1 and Vp2, respectively. Here,the output voltage Vout is still kept higher than the input voltage Vin,and thus, in each of the transistors P1 and P2, the gate potential (Vin)is lower than the source potential (Vout). Thus, the transistors P1 andP2 are continuously kept on, regardless of the change in the gatevoltages described above. As a result, an electric charge stored in thesmoothing capacitor Cex is discharged quickly via the transistor P1 tothe node to which the input voltage Vin is applied, and thus the outputvoltage Vout falls abruptly (from time t1 to time t2).

Thus, applying the input voltage Vin to each of the gates of thetransistors P1 and P2 allows the output voltage Vout to fall abruptlywhen the switching power supply IC 21 is turned off. In addition, withthis configuration, it is possible to prevent the voltage Vx appearingat the external terminal T2 from abruptly rising at the same time thatthe switching power supply IC 21 is turned off as described above andthereby to eliminate the need to provide the switching power supply IC21 with an unnecessarily large withstand voltage margin.

Then, when the output voltage Vout drops to the input voltage Vin attime t2, the transistors P1 and P2 are turned off. This makes itpossible to cut off the current path from the external terminal T2 tothe light emitting diode line LED, and hence, from this time on,regardless of a change in the input voltage Vin or other events, it ispossible to properly prevent a leak current from flowing into the lightemitting diode line LED. In addition, with this configuration, noexternally fitted transistor is needed to be provided for the purpose ofcutting off a leak current (see FIG. 6B), and thus neitherminiaturization nor weight/thickness reduction of the mobile-phone unitas a whole is prevented. After time t2, the output voltage Vout islowered to 0 [V] in about 10 [ms] by an internal pull-down circuit whichis not illustrated.

The above described transistor P2 is provided as means for cutting offthe current path via the body diode BD1 that accompanies the transistorP1. In a configuration where, in order to improve the switchingcharacteristic of the transistor P1, simply the backgate and the sourceof the transistor P1 are connected to each other, there exists a currentpath from the inductor Lex via the body diode BD1 to the light emittingdiode line LED. In contrast, with the configuration where the transistorP2 is arranged on the current path from the inductor Lex via the bodydiode BD1 to the light emitting diode line LED and both of thetransistors P1 and P2 are turned off when the operation of the switchingpower supply IC 21 is stopped, it is possible to cut off the currentpath described above and thereby to prevent a leak current from flowingfrom the inductor Lex to the light emitting diode line LED.

As for the transistor P3, the gate potential thereof (output voltageVout) falls abruptly from time t1 to time t2 as described above, andalso after the transistors P1 and P2 are turned off at time t2, the gatevoltage continues to decrease gradually. In contrast, the source voltageVy of the transistor P3, though it falls abruptly along with the outputvoltage Vout from time t1 to time t2, is kept at the same level as thevoltage Vx (input voltage Vin) after time t2, because it is pulled up bythe external terminal T2 via the body diode BD1. Thus, the transistor P3is kept off until the output voltage Vout falls to approximatelyVin-V_(GS)1.0 [V], and when the output voltage Vout falls to the abovementioned potential level at time t3, the transistor P3 spontaneouslyturns on without any requiring any control. Thus, it can be said thatthe transistor P3 serves as means for fixing, when the output voltageVout falls to the above mentioned potential level, the backgatepotentials of the transistors P1 and P2 at the input voltage Vin whichis the highest potential at the time. With this configuration, it ispossible to bias the backgates of the transistors P1 and P2 at a highpotential and thereby to ensure that the transistors P1 and P2 remain inthe off state.

What is important here is that, during the period from time t1 to timet2, when the transistors P1 and P2 are on (that is, when the chargestored in the smoothing capacitor Cex is quickly discharged), thetransistor P3 is kept off and the backgates of the transistors P1 and P2are each kept in a float state. Through achievement of such operation,it is possible to prevent a large current from flowing via thetransistor P3 into the internal circuits to which power is fed via theexternal terminal T1 and thereby to protect their components frombreakdown.

The embodiment described above deals with a configuration where, whendevice operation is stopped, the input voltage Vin is applied to thegates of the transistors P1 and P2, but it should be understood that thepresent invention may be practiced in any manner other than specificallydescribed above, and a configuration may be adopted where, as shown inFIG. 4, the output voltage Vout is applied to the gates of thetransistors P1 and P2 when the operation of the device is stopped.

With this configuration, when a shut-down instruction is given to theswitching power supply IC 21 at time t1, the transistors P1 and P2 areturned off. As a result, in this configuration, as compared with in theconfiguration of the above described embodiment, there exist a smallernumber of current paths through which the charge stored in the smoothingcapacitor Cex is discharged, and this allows the output voltage Vout todecrease gradually. Thus, in cases where it is necessary to turn on/offthe switching power supply IC 21 many times, adopting this configurationmakes it possible to reduce power loss in and improve efficiency of theswitching power supply IC 21.

However, in this configuration, where the transistors P1 and P2 areturned off at the same time that the switching power supply IC 21 isshut down, electric energy stored in the inductor Lex may cause thevoltage Vx appearing at the external terminal T2 to abruptly rise at thesame time that the switching power supply IC 21 is shut down asdescribed above. Hence, when this configuration is adopted, it isnecessary to provide the switching power supply IC 21 with an adequatelylarge withstand voltage margin.

Finally, with reference to FIG. 5, a detailed description will be givenof the output-short protection operation of the switching power IC 21configured as described above.

When the switching power supply IC 21 starts its operation in responseto an enable signal fed from outside at time t4, the output voltage Voutstarts to increase gradually at a predetermined rate. The waveforms inFIG. 5 result from a voltage of 3.6 [V] being applied as the inputvoltage Vin from before time t4.

The comparator CMP2 compares the output voltage Vout with apredetermined threshold voltage (a division voltage generated bydividing the input voltage Vin and derived from the connection nodebetween the resistors R2 and R3) and outputs as its comparison output avoltage signal Vb whose logic level (high level/low level) variesaccording to which of the output voltage Vout and the threshold voltageis higher. More specifically, when the output voltage Vout is higherthan the threshold voltage, the logic level of the voltage signal Vb ishigh, and when the output voltage Vout is not higher than the thresholdvoltage, the logic level of the voltage signal Vb is low. Thus, bymonitoring the logic level of the voltage signal Vb, it is possible todetect whether or not an output short (ground short) is occurring.

However, immediately after the start of the operation of the switchingpower supply IC 21 is the output-undetectable period of the comparatorCMP2 itself, during which the logic level of the voltage signal Vb iskept low although the output voltage Vout rises. Thus, if the justmentioned voltage signal Vb itself is used as the output-short detectionsignal (a signal for indicating to the switching control section CTRLwhether or not an output short is occurring), an output short iserroneously detected during the output-undetectable period.

To cope with this inconvenience, in the switching power supply IC 21 ofthis embodiment, the following signal processing (output-detectionmasking operation) is performed to avoid the above described erroneousdetection of an output short.

In the switching power supply IC 21 of this embodiment, simultaneouslywith the start of the operation of the switching power supply IC 21described above, a startup signal Sstart for controlling the switchingof the switch device SW is inputted (in FIG. 5, illustrated as a shiftto high level), whereby the switch device SW is turned on and thetransistor N3 is turned off. Incidentally, the transistor N3 isdischarge means of the capacitor C1 which is turned on when the startupsignal Sstart is not inputted.

The switching described above starts the charging of the capacitor C1,and the voltage signal Va applied to the gate terminal of the transistorN2 starts to increase gradually. Thereafter, until a voltage signal Vareaches a predetermined threshold voltage Vth, the transistor N2 is keptoff and a voltage signal Vc (output-detection masking signal) derivedfrom the drain of the transistor N2 is kept high.

The length of time the voltage signal Va takes to reach the thresholdvoltage Vth (from time t4 to time t5) corresponds to theoutput-detection masking period and is set longer than theoutput-undetectable period of the comparator CMP2. The length of theoutput detection masking period can be adjusted easily by appropriatelysetting the value of the constant current generated at the constantcurrent supply I1 or the value of the capacity of the capacitor C1.

The OR circuit OR feeds the switching control section CTRL with, as anoutput-short detection signal Vd, a voltage signal generated byperforming an OR operation between the voltage signals Vb and Vc. Thus,the logic level of the output-short detection signal Vd is high when atleast one of the voltage signals Vb and Vc is high, and the logic levelof the output-short detection signal Vd is low only when both of thevoltage signals Vb and Vc are low.

Here, during the period until the voltage signal Va reaches thethreshold voltage Vth (from time t4 to time t5), the level of thevoltage signal Vc is high as described above. Therefore, during theperiod, even if the level of the voltage signal Vb is kept low due tothe output detection incapability of the comparator CMP2, the logic ofthe voltage signal Vb is masked by the voltage signal Vc which is high,and thus the logic level of the output-short detection signal Vd ishigh. Thus, the switching control section CTRL, receiving theoutput-short detection signal Vd, recognizes that no output short isoccurring and performs its normal operation, and thus failure to startup can be prevented.

Then, charging of the capacitor C1 proceeds, and when the voltage signalVa reaches the threshold voltage Vth at time t5, the transistor N2 isturned on, and the level of the voltage signal Vc derived from the drainof the transistor N2 is turned to be low (approximately groundpotential). In this state, at time t5, the capacitor CMP2 is capable ofdetecting an output, and the level of the voltage signal Vb is high inresponse to the rise of the output voltage Vout. Thus, the logic levelof the output-short detection signal Vd generated at the OR circuit ORis high reflecting the logic state of the voltage signal Vb, and theswitching control section CTRL, having received the output-shortdetection signal Vd having the just mentioned logic, recognizes that nooutput short is occurring and thus can perform its normal operation.

If an output short (ground short) occurs at time t6, the output voltageVout falls to the ground potential, and hence the level of the voltagesignal Vb is turned to be low. In this state, the switch device SW iskept on by being fed with the start signal Sstart; the voltage signal Vacontinues to be kept higher than the threshold voltage Vth; and thetransistor N2 is kept on. Thus, the voltage signal Vc continues to bekept low. As a result, both of the voltage signals Vb and Vc are low,and thus the logic level of the output-short detection signal Vdgenerated at the OR circuit OR is turned to be low. Therefore, theswitching control section CTRL, receiving the output-short detectionsignal Vd having the just mentioned logic, recognizes that an outputshort is occurring and turns the transistors N1, P1, and P2 off, andthus can stop the operation of the device. The shut-down controldescribed above makes it possible to prevent a short-circuit currentfrom flowing from the inductor Lex to the switching power supply IC 21and thereby to protect devices inside the IC and components externallyfitted to the IC from breakdown.

In the embodiment described above, a configuration is dealt with wherethe charging operation of the capacitor C1 is used to generate anoutput-detection masking signal, but the configuration of the presentinvention is not limited to this configuration, and for example, a timercircuit or the like may be used to achieve the output-detection maskingoperation described above.

In the embodiment described above, it is assumed that the presentinvention is applied to the DC/DC converter 20 which is mounted in amobile-phone unit and is used as means for converting the output of thebattery 10. This, however, is not meant to limit the application of thepresent invention to such an embodiment; in practice, the presentinvention finds a wide application in voltage step-up type switchingpower supply devices in general.

The present invention may be practiced with various modifications andvariations made otherwise than specifically described above within thespirit of the invention.

INDUSTRIAL APPLICABILITY

The present invention offers an art that is useful in achievingminiaturization and improved reliability of an electronic deviceincorporating a voltage step-up type switching power supply device andthat is suitably applicable to electronic devices in generalincorporating a switching power supply device such as battery-operatedelectronic devices.

1. An output device integrated into an integrated circuit, the outputdevice comprising: an input terminal to receive an input voltage appliedvia an inductor; an output terminal to feed an output voltage to a load;an output transistor connected between the input terminal and apredetermined standard voltage node; a first transistor connectedbetween the input terminal and the output terminal; a second transistorconnected between the output terminal and a backgate of the firsttransistor; and switching control circuitry to control switching of theoutput transistor, the first transistor, and the second transistor,wherein, the output device is operable so that when the input voltage isstepped up to generate the output voltage, the switching controlcircuitry controls switching of the output transistor and the firsttransistor complementarily to each other, while keeping the secondtransistor on, and wherein when operation of the device is stopped, theswitching control circuitry turns off the output transistor, the firsttransistor, and the second transistor.
 2. The output device of claim 1further comprising: a power supply terminal to receive the input voltagedirectly; and a third transistor whose source is connected to thebackgate of the first transistor and whose drain is connected to thepower supply terminal, a gate of the third transistor being connected tothe output terminal.
 3. The output device of claim 1 wherein whenoperation of the device is stopped, the switching control circuitry isoperable to apply the input voltage to gates of the first and secondtransistors.
 4. The output device of claim 2 wherein when operation ofthe device is stopped, the switching control circuitry is operable toapply the input voltage to gates of the first and second transistors. 5.The output device of claim 1 wherein when operation of the device isstopped, the switching control circuitry is operable to apply the outputvoltage to gates of the first and second transistors.
 6. The outputdevice of claim 1 further comprising: an error amplifier to generate anerror voltage signal by amplifying a difference between a predeterminedtarget voltage and a feedback voltage that varies with the outputvoltage; an oscillator to generate a predetermined triangular-wavevoltage signal; and a PWM comparator to compare the error voltage signalwith the triangular-wave voltage signal and to generate a PWM signal,wherein, the output device is operable so that when the input voltage isstepped-up to generate the output voltage, the switching controlcircuitry, based on the PWM signal, controls the switching of the outputtransistor and the first transistor complementarily to each other. 7.The output device of claim 1 further comprising: an output-shortdetecting circuit to monitor the output voltage and to generate anoutput-short detection signal for indicating to the switching controlcircuitry whether or not an output short circuit is occurring, whereinthe switching control circuitry is operable to turn off the outputtransistor, the first transistor, and the second transistor when, basedon the output-short detection signal, the switching control circuitryrecognizes that an output-short is occurring.
 8. The output device ofclaim 7 wherein the output-short detecting circuit includes: acomparator to invert an output logic thereof when the output voltagefalls below a predetermined threshold voltage; and output detectionmasking means that, during a predetermined output detection maskingperiod starting at startup of the device, produces a constant outputregardless of the output logic of the comparator, and after thepredetermined output detection masking period ends, reflects the outputlogic of the comparator by inverting the output logic of theoutput-short detection signal, and wherein the switching controlcircuitry, upon recognizing that an output short is occurring based onthe output-short detection signal, turns off the output transistor, thefirst transistor, and the second transistor.
 9. An electronic devicecomprising: an output device operable to convert an output of a devicepower source, wherein the output device is integrated into an integratedcircuit and comprises: an input terminal to receive an input voltageapplied via an inductor; an output terminal to feed an output voltage toa load; an output transistor connected between the input terminal and apredetermined standard voltage node; a first transistor connectedbetween the input terminal and the output terminal; a second transistorconnected between the output terminal and a backgate of the firsttransistor; and switching control circuitry to control switching of theoutput transistor, the first transistor, and the second transistor,wherein, the output device is operable so that when the input voltage isstepped up to generate the output voltage, the switching controlcircuitry controls switching of the output transistor and the firsttransistor complementarily to each other, while keeping the secondtransistor on, and wherein when operation of the device is stopped, theswitching control circuitry turns off the output transistor, the firsttransistor, and the second transistor.
 10. The electronic device ofclaim 9 wherein the device power source is comprises a battery.